Fair Value Measurements - Schedule of Fair Value of Financial Assets Measured on Recurring Basis (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Financial Assets
Total financial assets	$ 133,623	$ 99,979
Money Market Funds [Member]
Financial Assets
Total financial assets	133,448	99,792
Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	175	187
Level 1 [Member]
Financial Assets
Total financial assets	133,623	99,979
Level 1 [Member] | Money Market Funds [Member]
Financial Assets
Total financial assets	133,448	99,792
Level 1 [Member] | Restricted Money Market Funds [Member]
Financial Assets
Total financial assets	$ 175	$ 187